In my earlier post I argued that the 'thin conception' of existence does not involve any obvious circularity. We define the verb 'exists' in terms of the following equivalence
"An American philosopher exists" is(def) equivalent to "some philosopher is American"
The verb 'exists' only appears on the left hand side of the definition, and so the definition is not circular, by the definition of circularity. To the objection that the right hand side has an elided adjective 'existing', i.e. that the definition should really be
"An American philosopher exists" is(def) equivalent to "some existing philosopher is American"
I appealed to what I will call the 'enlargement principle'. This is the principle that any adjective or qualifying term must, if it is to be meaningful, enlarge the conception signified by the term that it contracts. Thus 'blue' enlarges the concept signified by 'buttercup' when we attach the terms to form the composite 'blue buttercup'. Thus it is meaningful. But 'existing' does not enlarge the concept signified by 'American philosopher', for the sentence 'some philosopher is American' already states that an American philosopher is existing.
Against. Consider the concept signified by 'character in War and Peace'. There are hundreds of such characters. Some of these, such as Napoleon and the Czar of Russia, and the Russian general whose name I have forgotten, are or were historical characters. So they are existent in some sense (i.e. they used to exist, or exist in the afterlife). Others, such as Prince Bolkonsky or Natasha, never existed at all. So the contracting term 'existing' or 'historical' does enlarge the conception signified by the term it is attached to. And it does diminish the extension of the contracted term. The total number of characters in the book is large, but decreases significantly when we consider only the real historical ones. Thus the term 'existing' or 'historical' or 'real' is meaningful, and a real predicate. Thus the thin conception of existence does involve circularity.